So what’s the point of IQ tests? Well, they can predict lots of life outcomes. Here is a table from page 65 “The Scientific American Book of the Brain”. It shows the percentages of each IQ bracket and their life outcomes.
IQ is a better predictor of job performance than lots of things. From Schmidt and Hunter, they looked at the correlation between job performance ratings given by coworkers with various metrics to figure out what best predicts subjectively assessed work performance. The best prediction was a work sample test, followed by a structured interview and IQ test:
They also did a review of job performance studies :
Tarmo Strenze reviewed a series of longitudinal studies that compared how various life factors correlated with education level, income and occupation. The average sample size for each group is 97,083, which is the largest average sample size in any psychological study I have ever seen.
In all three groups, IQ was a better predictor for education level, occupation level, and income than any other factor.
Similarly, In the paper “Intelligence and School Grades: A meta-analysis”, Bettina Roth and co did a meta-analysis of data from more than 200 samples totaling 105,185 students, and shows that IQ tests strongly correlated with grades at 0.54.
So even if you are not convinced that IQ measures “intelligence”, it obviously measures something, and this thing is of practical importance. That said, there are some reasons to believe that IQ tests measure intelligence.
IQ Probably Measures “Intelligence”
We know that IQ tests measure intelligence because IQ tests correlate with peer and self rated intelligence. For instance, in Denissen et al. 2011 489 college students were divided into 20 groups which studied together for a period of one year. At the end of this year, subjects were asked to rate how intelligent their group mates were on a 7 point scale ranging from “not intelligence” to “very intelligent”. It was found that the better a subject did on an IQ test the smarter their group mates thought they were.
Palhusand Morgan 1997 found similar results and also showed that the correlation between peer rated intelligence and IQ increased the longer the peer knew the person being tested. They had 5 group discussion sections, and in the first section, found that in the first session, intelligence ratings were almost entirely a function of how much people talked. By the Fifth session, it was almost entirely a function of the person’s IQ – so talking a lot only increases perceived “intelligence” above IQ in the short term.
A significant body of research has also shown that IQ tests predict how intelligent people rate themselves as being (Paulhus, Lysy, and Yik 1998, ,Angelo and James 1977, and Reilly and Mulhern 1995). Clearly then, the smarter a person thinks they are, and the smarter their friends think they are, the better they tend to do on IQ tests.
So in summary, IQ tests predict life outcomes better than several factors commonly recognized to predict life outcomes, such as what your parents are like and how good your grades are. And IQ predicts one’s subjective perception of a person’s intelligence the longer you interact with them.